Long-term total OH reactivity measurements in a boreal forest

Abstract

Abstract. Total hydroxyl radical (OH) reactivity measurements were conducted at the second Station for Measuring Ecosystem–Atmosphere Relations (SMEAR II), a boreal forest site located in Hyytiälä, Finland, from April to July 2016. The measured values were compared with OH reactivity calculated from a combination of data from the routine trace gas measurements (station mast) as well as online and offline analysis with a gas chromatographer coupled to a mass spectrometer (GC–MS) and offline liquid chromatography. Up to 104 compounds, mostly volatile organic compounds (VOCs) and oxidized VOCs, but also inorganic compounds, were included in the analysis, even though the data availability for each compound varied with time. The monthly averaged experimental total OH reactivity was found to be higher in April and May (ca. 20 s−1) than in June and July (7.6 and 15.4 s−1, respectively). The measured values varied much more in spring with high reactivity peaks in late afternoon, with values higher than in the summer, in particular when the soil was thawing. Total OH reactivity values generally followed the pattern of mixing ratios due to change of the boundary layer height. The missing reactivity fraction (defined as the difference between measured and calculated OH reactivity) was found to be high. Several reasons that can explain the missing reactivity are discussed in detail such as (1) missing measurements due to technical issues, (2) not measuring oxidation compounds of detected biogenic VOCs, and (3) missing important reactive compounds or classes of compounds with the available measurements. In order to test the second hypothesis, a one-dimensional chemical transport model (SOSAA) has been used to estimate the amount of unmeasured oxidation products and their expected contribution to the reactivity for three different short periods in April, May, and July. However, only a small fraction (<4.5 %) of the missing reactivity can be explained by modelled secondary compounds (mostly oxidized VOCs). These findings indicate that compounds measured but not included in the model as well as unmeasured primary emissions contribute the missing reactivity. In the future, non-hydrocarbon compounds from sources other than vegetation (e.g. soil) should be included in OH reactivity studies.

The trend of the oxidants in boreal forest over 2007–2018: comprehensive modelling study with long-term measurements at SMEAR II, Finland

“…As pointed out in earlier publications (e.g. Boy et al, 2006;Praplan et al, 2019), carbon monoxide (CO)…”

“…At SMEAR II, monoterpenes are the dominant BVOCs (Bäck et al, 2012), and they are the major contributors to the OH reactivity of the measured organic compounds (Mogensen et al, 2015;Praplan et al, 2019). Thus, accurate modelling of monoterpenes is a crucial component for calculating the OH concentration.…”

“…SOSAA (a model to Simulate the concentrations of Organic vapours, Sulphuric Acid and Aerosol) is a 1-D chemistry transport model used to study the atmospheric composition inside the planetary boundary layer. In the past, SOSAA been applied to study characteristics of OH-reactivity (Mogensen et al, 2015;Praplan et al, 2019), oxidation of trace gases (Boy et al, 2013), emission of BVOCs (Smolander et al, 2013), vertical exchange and dry deposition of ozone (Zhou et al, 2017a) and…”

Answers to Anonymous Referee #1’s comments on “OH reactivity from different tree species: Investigating the missing reactivity in a boreal forest” by Praplan et al.

“…It is said that the studies of Sinha et al (2010) and Nölscher et al (2012) find missing reactivity in the boreal forest. Then you refer to Praplan et al (2019) who "recently demonstrated that including modelled oxidation products of VOCs that are not measured is not sufficient to explain the missing reactivity at the site". Please, make clear what site you are talking about here.…”

“…The reaction rate used in this study (1.2•10 −10 cm 3 s −1 , Atkinson et al, 1985) is, nevertheless, within the uncertainty of the value determined by Dillon et al, (1.28±0.1•10 −10 cm 3 s −1 ). While we did not discussed in detail uncertainty calculations in the original manuscript, we did in our previous publications (Praplan et al, 2017;2019), where we state that an uncertainty of 15 % is used for this reaction rate. p.5, l.122: The Comparative Reactivity Method (CRM) is used here for detecting the total OH reactivity with a gas chromatograph equipped with a PID.…”